Spatial variability of karst and effect on tunnel lining and water inflow. A probabilistic approach

摘要

Probabilistic methods can provide more insight and facilitate rationalized risk allocation when used in the framework of geotechnical baseline reports (GBR) by assessing the influence of a spatially variable distribution of cavities to tunnelling projects. A novel, probabilistic approach of the spatial distribution of karstic cavities is presented to assess both the risk of liner instabilities and water inflow when tunneling or mining through karstifled rock masses. The presented research work is based on the following geotechnical baseline statement: "There is a 10% probability of encountering a sediment-filled cavity, with a maximum size of 2 m(3)", which was assumed to be correct for the analysis. A Matlab script generates a random distribution of karst cavities, modelled as a circular shape, which is transferred into FEM 2D and 3D numerical analysis. The measured variables are: (a) the total discharge velocity (TDV) and (b) the index for capacity utilization of linings in tunnels (CULT-I), which act as a measure for liner stability assessment and water inflow rates, respectively. The approach can provide added value when used as a probability density function, in assessing the risk of a certain threshold to be reached, such as liner failure or a 50% increase in TDV values from baseline conditions. The model is most useful when analysed in 3D because the entire length of the tunnel alignment can be simulated. However, the approach is limited by the disproportional (to the anticipated results) computational effort required of analysing a 3D mesh and is restricted in applicability to the baseline statement assumption. The methodology presented provides a framework for further investigations that can be applied to the contractual requirements and specifications of a project and to varying baseline statements in order to to assess the risk from karst cavities to tunnelling works.